1. Field of the Disclosure
This specification relates to a cooling system of an emergency cooling tank with improved safety, which is capable of minimizing an increase in economical costs and maintaining a cooling function of the emergency cooling tank for a long term of time without refilling cooling water in the emergency cooling tank, by taking into account a characteristic of a quantity of heat transferred to the emergency cooling tank, as an ultimate heat sink, upon an occurrence of an accident of a nuclear reactor, and a nuclear power plant having the same.
2. Background of the Disclosure
An emergency cooling tank is used as a heat sink, which removes heat of a nuclear reactor upon an occurrence of an accident, in various types of nuclear reactors including an integral nuclear reactor. The heat of the nuclear reactor is ultimately transferred to the emergency cooling tank via a passive residual heat removal system (heat within the nuclear reactor) or a passive containment (building) cooling system (heat emitted into a containment). Accordingly, cooling water within the emergency cooling tank is evaporated such that the heat is emitted to the air.
A heat exchanger of the passive residual heat removal system is employing a water-cooling type (SMART nuclear reactor in Korea or AP1000 of Westinghouse Co. Ltd., in USA), an air-cooling type (SCOR in France), or a hybrid-cooling type (IMR in Japan) combining the water-cooling and the air-cooling.
In general, the water-cooling type heat exchanger has an advantage in fabrication of a heat exchanger with a small scale by virtue of excellent cooling efficiency. However, cooling water within the emergency cooling tank, to which the heat is transferred from the heat exchanger upon an occurrence of an accident, is gradually evaporated to be run out. Accordingly, the cooling water in the emergency cooling tank has to periodically be refilled for long-term cooling exceeding a cooling water storage capacity.
On the other hand, the air-cooling type heat exchanger does not have an emergency cooling tank, accordingly, there is no need to periodically refill the cooling water. However, the air-cooling type heat exchanger exhibits lower cooling efficiency than the water-cooling type. The heat transfer efficiency of transferring heat to the outside (to the air) through the wall surface of the tube is low. The efficiency of the air-cooling type heat exchanger depends on heat transfer efficiency of a wall surface of a tube with which air comes in contact. Consequently, an increase in a size (capacity) of the heat exchanger is required.
Also, the hybrid-cooling type heat exchanger also exhibits a heat transfer performance which is decreased extremely lower than the water-cooling type at the time point of operating in an air-cooling manner. Thus, it requires for a greater size than the water-cooling type heat exchanger.
In order to cool an inside of the heat exchanger of the passive residual heat removal system, a condensation heat exchanger of a steam condensation type with excellent heat transfer efficiency is employed. Since the heat exchanger of the passive residual heat removal system is generally operating under high temperature and high pressure environments, design pressure thereof may be extremely high and economic feasibility is drastically lowered when the heat exchanger has an increased size.
The nuclear reactor does not always transfer constant heat upon an occurrence of an accident thereof. Unlike a typical boiler, the nuclear reactor generates residual heat from its core for a considerably long-term of time even after a shutdown of the core of the nuclear reactor. Accordingly, when the nuclear reactor is shut down due to an accident or the like, a large quantity of residual heat is emitted from the core at the beginning of the accident. As the time elapses, the emitted residual heat is drastically reduced. In turn, the heat transferred from the nuclear reactor into the emergency cooling tank is remarkably reduced according to the lapse of time after the occurrence of the accident.
In the related art emergency cooling tank, the emergency cooling tank has a top open due to an accident characteristic of the nuclear reactor. When heat is transferred to the emergency cooling tank upon an occurrence of an accident, the cooling water within the emergency cooling tank, to which the heat is transferred, is increased in temperature and evaporated so as to be changed into a phase of steam. The steam is externally emitted through the open top of the emergency cooling tank. Consequently, a heat load is treated by evaporation heat.
However, the related art structure had the problem that the cooling water within the emergency cooling tank is gradually reduced to be run out, due to a long-term operation of the emergency cooling tank. When the cooling water within the emergency cooling tank is depleted, the emergency cooling tank lost its function. Hence, unless it is refilled with cooling water in a periodic manner, there is a limitation in maintaining the function for a long term of time. Further, when the use of an electric power system for refilling the cooling water is stopped for an extended time upon an occurrence of an accident exceeding a design reference, the accident level might extend to a severe accident.